Add documentation for new GenericDocument ctor taking object type.

Update also documentation of the existing GenericDocument constructor.

include/rapidjson/document.h

@@ -1748,6 +1748,13 @@ public:
     typedef GenericValue<Encoding, Allocator> ValueType;    //!< Value type of the document.
     typedef Allocator AllocatorType;                        //!< Allocator type from template parameter.
 
+    //! Constructor
+    /*! Creates an empty document of specified type.
+        \param type             Mandatory type of object to create.
+        \param allocator        Optional allocator for allocating memory.
+        \param stackCapacity    Optional initial capacity of stack in bytes.
+        \param stackAllocator   Optional allocator for allocating memory for stack.
+    */
     GenericDocument(Type type, Allocator* allocator = 0, size_t stackCapacity = kDefaultStackCapacity, StackAllocator* stackAllocator = 0) :
         GenericValue<Encoding, Allocator>(type),  allocator_(allocator), ownAllocator_(0), stack_(stackAllocator, stackCapacity), parseResult_()
     {
@@ -1756,7 +1763,8 @@ public:
     }
 
     //! Constructor
-    /*! \param allocator        Optional allocator for allocating memory.
+    /*! Creates an empty document which type is Null. 
+        \param allocator        Optional allocator for allocating memory.
         \param stackCapacity    Optional initial capacity of stack in bytes.
         \param stackAllocator   Optional allocator for allocating memory for stack.
     */